4.7.1 ISOLATION OF PUMPING STATIONS FROM INCOMING FLOW
Sewage generally arrives at a pumping station via a gravity main. Irrespective of the type of pumping station, there are times when a pumping station must be wholly or partially closed down for maintenance or further works. This means that sewage must be prevented from entering the wet well. When only one wet well is provided in the pumping station, storage of sewage under shut down conditions must be considered. If shut down for an extended period is being proposed, tankering or bypassing with temporary pumping may be necessary.
4.7.2 ISOLATING PUMPING STATIONS WITH SUBMERSIBLE PUMPS
The arrangement used to cut off sewage flow to pumping stations with submersible pumps is shown in the Typical Sewage Pumping Station Arrangements contained in Appendix 4D. The knife gate valve is used as due to the small face-to-face dimensions it does not protrude greatly into the pumping station wet well itself and is suitable for use with raw sewage. An extension spindle allows operation of the valve from the top of the pumping station.
4.7.3 ISOLATION OF PUMPS FROM THE RISING MAIN AND WET WELL
Each pump is fitted with a reflux valve and a stop valve on the delivery side of the pump. The reflux valve ensures that backflow, from the rising main through the pump, does not occur when the pump is not operating. The stop valve is positioned beyond the reflux valve so that the rising main can be isolated when it is necessary to carry out maintenance on either the pump or the reflux valve. The stop valve should be in the fully open position when the pumps are operating to ensure minimum resistance to flow.
The stop valve does, however, allow the initial filling of the rising main from the pumping station. By opening the valve slowly, the initial lack of friction head, due to there being no sewage in the main, can be overcome thus ensuring that too great an electrical current is not drawn by the pump motor.
Submersible motor pumps do not usually require separate isolation as they may be simply withdrawn from the well for inspection, removal of chokes and maintenance.
In the case of single well pumping stations with submersible pumps, valves on the discharge pipework are to be mounted in a separate pit from the wet well. Generally this pit is small and shallow relative to the main wet well. This allows the designer to minimise the size of the wet well and makes operation and maintenance of valves easier to carry out. In the event of flooding of the wet well, solids would also tend to hang up around the valves. The separate valve pit also allows a suitable accessible point for the attachment of pressure gauges to check the performance of the pumps.
4.7.4 TYPES OF VALVES USED
Isolating Valves
Valves for isolating duties should be gate valves because they offer unobstructed full-way flow and are capable of shearing fibrous matter upon closure. Conventional gate valves to AS 2638 are used on the delivery side, however, stainless steel knife gate valves are required on the inlet.
Non-Return Valves
The preferred valve is a long body swing check type, again because they offer unobstructed full-way flow when open and they have the ability to shear fibrous matter upon closure. Only double hinged swing check valves are used as single hinged are more prone to stay partially open when any solids rest on the sealing face. By use of an extended spindle (hinge pin on the body), no-flow sensing can be achieved with a cam operated micro-switch or electronic sensor. Addition of a counterweight enables control of the closing characteristics for water hammer control purposes.
Swing check type non-return valves installed in the vertical up position should be avoided because deposition of sediment on the closed disc can prevent the valve from opening. It is also necessary that the gate does not open past the vertical position and thus stick open. A counterweight may be necessary to ensure closure and thus head losses will be increased. Where it is necessary to install a non-return valve in the vertical up position, a sinking ball type valve should be used since this type of valve is able to shed sediment. Electromagnetic proximity switches are available with ball type valves for no-flow sensing. Care should be taken with the ordering of ball type non-return valves as they may be used in the vertical, horizontal or in between orientation. The design of the ball is, however, specific to each case (ie. the specific gravity is varied depending upon orientation) and will not operate
satisfactorily unless the correct ball is selected.
Non-return valves are installed between the pump and its discharge isolating valve to facilitate maintenance of the non-return valve whilst operating other pumps in the pumping station.
Materials of Construction
Materials used are described in Hunter Water Corporation standard technical specifications STS 402 and STS 103.
4.7.5 VALVE PITS
Particular attention must be paid to sizing of valve pits to ensure, that as well as fitting valves and proper dismantling provisions, there will be sufficient room to attach pressure gauges and, where required, install monorail ladder.
As a guide to designers of small and medium submersible pumping stations Figures 13 and 14 show the minimum clearances and dimensions required for typical valve pits.
4.7.6 PIPEWORK
Inlet pipework from the collecting access chamber to station wet well shall be DICL.
Nominal fall shall be 100 mm. The connecting pipework shall be sized for ultimate flow but shall not be less than the diameter of the largest sewer discharging into the collecting access chamber.
Discharge pipework from the pump discharge bend through the valve pit to rising main chainage 0.0 shall be DICL.
DICL pipes in ground shall be protected from external corrosion by a loose polyethylene sleeving
Two (2) flexible joints shall be provided for all pipes connected to structures to protect against differential settlement. The first joint shall be at the edge of the structure or as close as possible to the structure. The second joint shall be approximately 1500 mm from the first joint.
4.7.7 VELOCITIES IN DISCHARGE VALVES AND PIPEWORK
The discharge pipework is normally selected with a smaller diameter than the rising main with velocities above 2 m/s being preferable to ensure satisfactory operation of reflux valves and avoid sedimentation in the valves and vertical pipework, for example, in the case of a DN 150 rising main with velocity of 1.2 m/s, DN 100 valves and discharge pipework with a velocity of 3 m/s would be selected. The maximum velocity is limited to 4 m/s to avoid scouring of the pipework lining or material.